Low health warning effect script example for Unity.

CameraEffect.cs

using UnityEngine;

[ExecuteInEditMode]
[RequireComponent(typeof(Camera))]
public class CameraEffect : MonoBehaviour
{
    public Material material;

    private void OnRenderImage(RenderTexture source, RenderTexture destination)
    {
        if (material == null)
        {
            Graphics.Blit(source, destination);
            return;
        }

        Graphics.Blit(source, destination, material);
    }
}

Health.cs

using UnityEngine;

public class Health : MonoBehaviour
{
    public float CurrentAmount
    {
        get
        {
            return _currentAmount;
        }
        set
        {
            _currentAmount = value;
            UpdateCameraEffectBasedOnHealth();
        }
    }
    private float _currentAmount;

    [SerializeField]
    private CameraEffect cameraEffect = null;

    [SerializeField]
    private float maxAmount = 100f;

    [SerializeField]
    private Material lowHealthEffect = null;

    private void Start()
    {
        CurrentAmount = maxAmount;
    }

    private void UpdateCameraEffectBasedOnHealth()
    {
        bool healthIsLessThanHalf = CurrentAmount < maxAmount / 2f;
        
        if (healthIsLessThanHalf)
            cameraEffect.material = lowHealthEffect;
        else
            cameraEffect.material = null;
    }
}